Well I use rubber (elastic) bands, which I know you've tried and apparently don't like. I just stuff them in the pockets of the campaign bag (or whatever it was called).

My kids and I purchased plastic card boxes for our Pokemon cards a few years back and those would probably work, but maybe would not hold overlord decks. I don't know what specific boxes they are, but surely you can buy boxes from the same place you purchased the sleeves. That's what we did.

One cannot really get a proper shuffle when the cards are in sleeves (and I do not know how anyone can do this). By this I mean splitting them into two decks and shuffling them that way.

The only way I could shuffle the cards when in sleeves was by holding them in one hand and performing an under over movement of the cards from top to bottom.

We played one game when I did this type of shuffle and my friend noted after he looked over his game report that the same pattern of cards showed up in about the same order before and after the shuffle.